1. Field of the Invention
The invention concerns a method to reduce movement artifacts in magnetic resonance images, as well as a magnetic resonance apparatus to implement the method.
2. Description of the Prior Art
Magnetic resonance imaging of the heart presents a particular difficulty that must be overcome, because cardiac movement is still overlaid by the breathing movement. In order to reduce artifacts due to the cardiac movement, segmented acquisition techniques are used. The patient is additionally required to hold his or her breath during the data acquisition. The required breath-hold times are in the range of 20 seconds. Particularly for cardiac patients, it is often not possible for the patient to hold his or her breath for such a long period of time. This leads to inconsistent raw data (k-space data) in segmented acquisitions. The reconstructed images then have artifacts that prevent a clinical interpretation. Depending on the k-space sampling scheme or reordering scheme, the artifacts in the image manifest in the phase coding direction as “ghosting” or even as “blurring/ringing” at the boundaries of contrast jumps. In applications in which a transient contrast ratio is imaged, the discrete ghost structures can superimpose further distant structures. For example, this occurs in morphological images that were acquired with a “dark blood” turbo spin echo technique. Even in the case of exposures in which small contrast changes in a transient contrast ratio are diagnostically important—for example activity examinations with “Late Gadolinium Enhancement” or edema imaging with T2 contrast—movement artifacts prevent a reliable diagnosis. The aforementioned two imaging techniques are the most common clinical cardiac examinations and must therefore be designed to be as robust as possible.
In order to avoid the movement artifacts described above, which are due to an insufficient breath-hold length of time during the data acquisition, the acquisitions are (for example) repeated until image data without movement artifacts are present. Unsegmented image data are also generated, but must have a lower quality than the segmented image data.
In the data acquisition, cushions are also used that increase the clearance of the local antennas from the imaging area. However, this is also at the cost of the signal-to-noise ratio, and therefore of the image quality.
To improve the image quality, coil arrays are increasingly used that have a high signal-to-noise ratio. 32 to 128 channels are the state of the art. The local antennas required for measurement are connected with the signal processing channels of the magnetic resonance apparatus by means of a hardware switching matrix, or also purely electronically. The received nuclear magnetic resonance signals from multiple local antennas can thereby be combined for the purpose of reducing the required reception channels.
A method for digital channel reduction in MR reception systems is described in E 10 2009 012 109 A1 (corresponding to US 2010/0225317 A1). The weighting factors that are required for combination are determined with low resolution from a preceding measurement. The resulting combination is designated therein as an expanded software version of an already known hardware mode matrix. The method described therein allows a reduction of the number of channels, while still achieving an optimal signal-to-noise ratio in a selected imaging region. The imaging region can be predetermined either by a user, or automatically.
DE 690 25 513 T2 concerns an arrangement for image generation by means of magnetic resonance, wherein a fast measurement of dynamic processes should take place given a high signal-to-noise ratio. For this purpose, a number of surface coils is used that can simultaneously acquire magnetic resonance signals. The magnetic resonance system described therein also has an aliasing integration device and a compositing or synthesizing device in order to produce a weighting/additive processing of the acquired magnetic resonance signals, so as to combine the multiple acquired magnetic resonance signals. The image data of all surface coils are pixel-by-pixel in order to obtain a single final image. The signal readout in this known procedure can take place with a time offset within a scan interval if there is a low correlation between the surface coils.
U.S. Pat. No. 4,825,162 discloses a method in order to simultaneously obtain a different magnetic resonance response signal from each of a number of surface coils arranged near one another. Each of these magnetic resonance response signals is combined with a separate image of the sample, which are then in turn combined point for point in order to obtain a single composite magnetic resonance image of the entire sample. It is assumed that the surface coils have no interaction among one another whatsoever. In the image reconstruction, the individual images are combined with a weighting such that a target function is maximized in order to obtain an improved SNR of the entire reconstruction image.
These two publications thus concern the reconstruction of the entire magnetic resonance image from the data of all local antennas.